As shown in FIG. 9, there has been known a refrigeration cycle apparatus (JP 2001-116371 A) that is intended to improve COP (coefficient of performance) by connecting an expander 4 to a compressor 3 with a shaft so as to utilize the power obtained by the expander 4 for driving the compressor 3. Since the expander and the compressor are connected to each other with the shaft in this refrigeration cycle apparatus, the ratio of density ρe of the refrigerant at an inlet of the expander to density ρc of the refrigerant at an inlet of the compressor, which is ρe/ρc, always is constant (constraint of constant density ratio). Accordingly, the COP is poor under operational conditions straying from ideal design conditions.
Facing this problem, there has been proposed a refrigeration cycle apparatus (JP 2004-212006 A) having an expander 23, a first compressor 21, and a second compressor 22 as shown in FIG. 10. The first compressor 21 and the expander 23 are connected to each other with a shaft, and the second compressor 22 is provided in parallel with the first compressor 21. In this refrigeration cycle apparatus, it is possible to avoid the constraint of constant density ratio by controlling a rotation speed of the second compressor 22. As a result, the COP can be kept high all the time.
A defrosting operation can be mentioned as one of the operation modes essential for refrigeration cycle apparatuses. When the outside air temperature is closer to 0° C., frost starts to form on an evaporator, lowering extremely the efficiency of the heat exchange at the evaporator. Thus, a high temperature refrigerant is made to flow through the evaporator to melt the frost. A cycle of this defrosting operation (hereinafter also referred to as a defrosting cycle) is as follows.
FIG. 11 is a Mollier diagram of refrigeration cycle apparatuses (FIG. 9 and FIG. 10) in which power is recovered by an expander during the defrosting operation. The refrigerant is compressed by a compressor from Point a1 to Point b1 and flows into a radiator. When the radiator is a water-refrigerant heat exchanger, the flow rate of water flowing into the radiator is zero, and thus the refrigerant flows into the expander almost without a change in enthalpy from Point b1. Thereafter, the refrigerant is decompressed by the expander and moves to an inlet (Point c1) of the evaporator. Since power is recovered at the expander, the enthalpy of the refrigerant at Point c1 is lower than that of the refrigerant at Point b1. The refrigerant that has flowed into the evaporator melts the frost by heating the evaporator, and then returns to an inlet (Point a1) of the compressor.
Next, FIG. 12 shows a Mollier diagram of a conventional refrigeration cycle apparatus using an expansion valve, during the defrosting operation. The refrigerant is compressed by a compressor from Point a2 to Point b2 and flows into a radiator. The refrigerant that has flowed into the expansion valve at Point b2 is decompressed without a change in enthalpy, and moves to an inlet (Point c2) of an evaporator. The refrigerant that has flowed into the evaporator melts the frost by heating the evaporator, and then returns to an inlet (Point a2) of the compressor.
In each of the defrosting cycles shown in FIG. 11 and FIG. 12, the amount of the heat that the evaporator obtains is equal to the difference between the enthalpy of the refrigerant at the inlet of the evaporator and the enthalpy of the refrigerant at the outlet of the evaporator. Since the power is recovered also during the defrosting operation, the enthalpy difference Δh1 in the defrosting cycle using the expander is smaller than the enthalpy difference Δh2 in the defrosting cycle using the expansion valve. Thus, the defrosting cycle using the expander needs a longer defrosting time than that of the defrosting cycle using the expansion valve.